Make it easy for projects to depend on libxrpl by adding an `ALIAS`
target named `xrpl::libxrpl` for projects to link.
The name was chosen because:
* The current library target is named `xrpl_core`. There is no other
"non-core" library target against which we need to distinguish the
"core" library. We only export one library target, and it should just
be named after the project to keep things simple and predictable.
* Underscores in target or library names are generally discouraged.
* Every target exported in CMake should be prefixed with the project
name.
By adding an `ALIAS` target, existing consumers who use the `xrpl_core`
target will not be affected.
* In the future, there can be a migration plan to make `xrpl_core` the
`ALIAS` target (and `libxrpl` the "real" target, which will affect the
filename of the compiled binary), and eventually remove it entirely.
Also:
* Fix the Conan recipe so that consumers using Conan import a target
named `xrpl::libxrpl`. This way, every consumer can use the same
instructions.
* Document the two easiest methods to depend on libxrpl. Both have been
tested.
* See #4443.
Introduces a conanfile.py (and a Conan recipe for RocksDB) to enable building the package with Conan, choosing more recent default versions of dependencies. It removes almost all of the CMake build files related to dependencies, and the configurations for Travis CI and GitLab CI. A new set of cross-platform build instructions are written in BUILD.md.
Includes example GitHub Actions workflow for each of Linux, macOS, Windows.
* Test on macos-12
We use the <concepts> library which was not added to Apple Clang until
version 13.1.6. The default Clang on macos-11 (the sometimes current
version of macos-latest) is 13.0.0, and the default Clang on macos-12 is
14.0.0.
Closes#4223.
